Page 404 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

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part III The Earth–Atmosphere Interface
0°
PACIFIC PLATE
20°
ATLANTIC OCEAN
20°
PLATE
PLATE
40°
JUAN DE FUCA PLATE
Tropic of Cancer
COCOS PLATE
Equator
EURASIAN PLATE
ARABIAN PLATE
40°
PHILIPPINE PLATE
PACIFIC PACIFIC PLATE
OCEAN
CAROLINE PLATE FIJI
Subduction zone Motion of plate
Spreading ridge offset by transform faults
160° 140° 120° 100° 80° 60° 40° 20° 0° 20° 40° 60° 80° 100° 120° 140° 160°
80°
NORTH AMERICAN PLATE
CARIBBEAN PLATE
80°
PACIFIC
OCEAN OCEAN
Tropic of Capricorn 40°
60°
Antarctic Circle
ROBINSON PROJECTION
INDO-AUSTRALIAN PLATE
60°
Fracture zone Transform fault
Convergent plate boundary—
plates converge, producing a subduction zone. Coastal area features mountains, volcanoes, and earthquakes.
Divergent plate boundary—
plates diverge in areas of seafloor spreading at mid-ocean ridges.
▲Figure 12.19 earth’s major lithospheric plates and their movements. Each arrow represents 20 million years of move- ment. The longer arrows indicate that the Pacific and nazca plates are moving more rapidly than are the Atlantic plates. Compare the length of these arrows with the purple areas on Figure 12.14. [Adapted from U.S. geodynamics Committee, na- tional Academy of Sciences and national Academy of Engineering.]
Animation
Motions at Plate Boundaries
Notebook
Plate Boundaries
Animation
Animation
Forming a Divergent Boundary
Animation
India Collision with Asia
Notebook
Plate Boundaries
NAZCA SOUTH AMERICAN
INDIAN PLATE 20°
Plate Asthenosphere
Plate
Plate
• Transform boundaries occur where plates slide past one another, usually at right angles to a seafloor spread- ing centre. These are the fractures stretching across the mid-ocean ridge system worldwide, first described in 1965 by University of Toronto geophysicist Tuzo Wilson. As plates move past each other horizontally, they form a type of fault, or fracture, in Earth’s crust, which is a transform fault. See the Visual Analysis at the end of this chapter for an illustration of transform faults.
Along these fracture zones that intersect ridges, a transform fault occurs only along the fault section that lies between two segments of the fragmented mid-ocean ridge (Figure 12.19). Along the fracture zone outside of
the transform fault, the crust moves in the same direction (away from the mid-ocean ridge) as the spreading plates. The movement along transform faults is that of horizon- tal displacement—no new crust is formed or old crust subducted.
The name transform was assigned to these features because of the apparent transformation in the direction of fault movement—these faults can be distinguished from other horizontal faults (discussed in Chapter 13) because the movement along one side of the fault line is opposite to movement along the other side. This unique movement results from the creation of new material as the seafloor spreads.
All the seafloor spreading centres on Earth feature these fractures, which are perpendicular to the mid-ocean
ANTARCTIC PLATE
AFRICAN PLATE
Oceanic trench
Mid-ocean ridge
Plate
Asthenosphere
Transform boundary—
plates slide past each other, forming a fracture zone including a transform fault in which plates move past each other in opposite directions; along the fracture zone outside of the active fault, plates move in the same direction.
0
1500
3000 KILOMETRES
Correlating Processes and Plate Boundaries